Why I can't receive SIGPOLL signal from ioctl function? - c

Question

I got a strange problem that I can't solve. This is my code.
#include <stdio.h>
#include <stropts.h>
#include <signal.h>
#include <sys/types.h>
void handle_signal(int s)
{
char c = getchar();
printf("got char '%c'\n");
if(c == 'q')
{
exit(0);
}
}
int main(int argc, char** argv)
{
sigset(SIGPOLL, handle_signal);
ioctl(0, I_SETSIG, S_RDNORM);
printf("type q to exit");
while(1);
return 0;
}
When I run this program, I type character in terminal but it did not work!!! I can not receive SIGPOLL signal. Have someone can give me some advice? By the way, my operating system is ubuntu 12.04.

Answer 1

On Linux it needs to set O_ASYNC flag and F_SETOWN property on the file descriptor to get SIGIO signal (a synonym of SIGPOLL). And the signal handler can only call async-signal safe functions:
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <ctype.h>
void handle_signal(int) { // Can only use async-signal safe functions here.
char msg[] = "got char c\n";
char* c = msg + (sizeof msg - 3);
if(1 != read(STDIN_FILENO, c, 1) || !isprint(*c))
return;
write(STDOUT_FILENO, msg, sizeof msg - 1);
if(*c == 'q')
exit(EXIT_SUCCESS);
}
int main() {
printf("type q to exit\n");
signal(SIGIO, handle_signal);
fcntl(STDIN_FILENO, F_SETFL, O_ASYNC | fcntl(STDIN_FILENO, F_GETFL));
fcntl(STDIN_FILENO, F_SETOWN, getpid());
sigset_t mask;
sigemptyset(&mask);
for(;;)
sigsuspend(&mask);
return EXIT_SUCCESS;
}
You may also like to have a look at F_SETSIG that allows receiving a signal of your choosing and extra information into the signal handler.